In an information processing system including an information processor, a communication processor and a server, for example a predetermined packet is periodically transmitted to a server from an information processor such as a PC (Personal Computer) for home use or a home appliance via a communication processor. Details of this technique are disclosed for example in the pamphlet (Page 1, FIG. 1 and the like) of the International Publication 2004/030292. The communication processor may be a router including the NAT (Network Address Translation) feature. The predetermined packet is periodically transmitted to maintain a port of a communication processor (to keep a packet from a WAN being transmitted to an information processor via a communication processor) in order to detect whether the IP address of the WAN (Wide Area Network) of the communication processor has changed or to make an access to the information processor from an external device such as a cell phone via a server.
In a communication processor including the NAT feature such as a router, the private IP address and the port number of a LAN (Local Area Network) are converted to the global IP address and the port number of a WAN when a packet is transmitted from the LAN to the WAN. In case a return packet is received from the WAN, reverse conversion is made and a resulting packet is passed to an information processor. The communication processor has time for such address conversion set therein. To be more precise, when a predetermined time has elapsed since an address conversion was last performed between a WAN and a LAN, the address conversion on a packet received from the WAN is no longer performed (address conversion is performed anew on a packet received from the LAN). That is, the packet from the WAN is not received by the information processor. This means that an external device such as a cell phone cannot access the information processor via a server. The predetermined period is hereinafter called the port keeping time.
In the above information processing system, the communication processor must be always ready to perform address conversion on a packet received from a server (from a WAN) in order for the information processor to receive information from the server. Thus, even in case it is unnecessary to exchange information between an information processor and a server, packets must be periodically transmitted from information processor to the server via a communication processor so as to enable the communication processor to perform address conversion on a packet transmitted from the server. A demand accompanies this practice that the transmission period of a packet to be periodically transmitted by the information processor should be as long as possible. This is to reduce transmission of unnecessary packets and the processing load on the information processor caused by transmission of packets. In particular, it suffices to shorten the transmission period of packets to be periodically transmitted by the information processor by a small amount (for example one or two seconds) with respect to the port keeping time of the communication processor to which the information processor is connected.
It is not known what type of communication processor the information processor will be connected to. In general, the period corresponding to the shortest port keeping time among those of the variety of communication processors available from manufacturers is set to the information processor and packets are transmitted using this period. In this case, even with the information processor connected to the communication processor having a long port keeping time, packets are transmitted in a preset short period, which results in transmission of numerous unnecessary packets.